The present invention relates to apparatus for intermittently moving webs of photographic material or the like. More particularly, the invention relates to improvements in apparatus for effecting rapid acceleration of a web of photographic material or the like from zero speed to a maximum speed and for thereupon effecting controlled deceleration of the web to zero speed. Such apparatus are needed in photographic copying machines to facilitate subdivision of a web consisting of a row of neighboring prints into discrete prints or to place successive frames of an exposed and developed film into accurate register with the optical system of a printer.
It is already known to advance a web of photographic material or the like by means of a variable-speed motor and to change the speed of the motor in response to detection of markers (e.g., notches or perforations) which are provided in the web. As a rule, the detection of a marker results in deceleration of the motor from a relatively high normal or first speed to a much lower second or creep speed, and the motor is thereupon arrested with a predetermined delay subsequent to deceleration to second speed. Such mode of decelerating the web to zero speed was considered necessary in order to reduce the influence of friction as well as the influence of inertia of moving parts including the web. A drawback of the just described apparatus is that the deceleration of a web from maximum speed to zero speed takes up too much time because each stoppage of the motor is invariably preceded by a certain interval during which the motor is driven at the relatively low and normally constant second speed. Moreover, and in order to insure that the web is invariably arrested in an optimum position (e.g., in a position in which the knife of a severing mechanism can separate two neighboring prints exactly across the center of the frame line between such prints), the second speed must be sufficiently low to enable the motor to come to a full stop with a high degree of reproducibility. Rapid transport of a web between successive stoppages (i.e., the achievement of short intervals between successive stoppages of the web) is highly desirable in modern high-speed machines for the processing of photographic material or the like.
German OS No. 2,246,543 discloses a modified apparatus which is intended to effect rapid deceleration of a web from maximum speed to zero speed. The apparatus comprises means (e.g., a tachometer generator) which furnishes a signal indicating the momentary speed of the motor (and hence the momentary speed of the web) and means for furnishing a signal which indicates the maximum permissible speed of the motor at any given stage of movement of a predetermined point of the web toward that position which the point should occupy when the web is brought to a full stop. This is intended to insure that the motor is operated at a maximum permissible speed during the entire stage of deceleration from maximum speed to zero speed without permitting the aforementioned point of the web to overshoot the intended position. A drawback of the just described apparatus is that the system for indicating the maximum permissible speed of the web during each stage of its deceleration to zero speed is extremely complex and prone to malfunction. The system includes a device which is mechanically coupled to the web and indicates the distance of the predetermined point of the web from the position which the point is to occupy when the web is brought to a full stop. The accuracy of such device is unsatisfactory, i.e., this device constitutes a systematic source of errors whose magnitude depends on the dissolution accuracy of the device, and this contributes to complexity of other parts of the system because they must compensate for inaccuracies of the distance measuring device. Slippage of the web relative to the customary advancing or transporting rolls which receive torque from the variable-speed motor also contributes to errors, especially since such slippage normally occurs during deceleration of the web which, in turn, causes the web to overshoot, or come to a full stop short of, its intended position.